Interferometer Calibration System And Method - Patent 8149420

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Interferometer Calibration System And Method - Patent 8149420 Powered By Docstoc
Description: BACKGROUND Optical interferometers are often used to make accurate measurements of position. For example, in semiconductor fabrication applications, a movable stage having a wafer mounted thereon must be accurately positioned with respect to an opticalsystem that projects a pattern image onto the surface of a wafer. Through photolithography, the pattern image defines the patterning of a constituent layer of a semiconductor device formed in a small region of the wafer. The pattern image has to bealigned very accurately with elements of the semiconductor device already formed in or on the wafer. The accuracy required of the positioning operation depends on the feature size of the pattern image. Decreasing feature sizes have driven a demand forever more accurate positioning mechanisms, and hence, metrology systems for accurately measuring the position of the stage. Interferometer-based position metrology systems are typically used to measure position. Interferometers typically measure displacement, i.e., a change of position. To obtain a measurement of position from a metrology system that measuresdisplacement, the stage is initialized to an accurately-known start position and is then moved to a current position. The metrology system measures the displacement of the current position from the start position, and adds the measured displacement tothe start position to obtain the current position of the stage. When the stage makes more than one movement to reach the current position, the measured displacements of all the movements are summed and the result is added to the start position to obtainthe current position. Recently, interferometer-based metrology systems of the types disclosed by Trutna, Jr. et al. in United States patent application publication no. 2007/0146722 and by Schluchter et al. in U.S. patent application Ser. Nos. 11/686,855 and12/172,810 have been used to measure the displacements of a movable object such as a stage. All of these disclosu